1. You could probably keep the amount of plants you have now, or add one or two more. (As a side note, live plants would be much better for your tank, as it helps the biological filtration and fish like them much more than plastic/silk ones!)
2. I've just always stuck with the natural green color, mainly because the neon colors annoy me and I'm sure the fish think they look just as stupid as me. Then again, I've just always wanted my fish to have as natural of a habitat as possible.
3. This is one I will really leave up to you. Aeration ornaments are usually just for looks; little to no oxygen actually diffuses into the water from it, and the splash from your filter is usually enough to keep oxygen in the water. But if looks is exactly the reason you want it, I suggest getting one to match the theme of your tank. Tanks with random types of decor tend to take away from its beauty.
4. I've always used a natural background because it seems to stress my fish out less and make them feel as if they're in their natural habitat. At times, however, one of my cory cats seems confused as to why he can't go into the piece of driftwood in my background! lol.